The invention relates to nozzles used in furnace apparatus. While the invention has particular application to chemical recovery units it also has application to other slagging furnaces and other furnace apparatus. Although the chemical recovery boiler shares its general appearance and many of its physical components with power-boilers, it is unique in the power-generation field. Such boilers have a three purposes. The first is to reduce the sulfur compounds in the black liquors to sodium sulphide. The second is to recover inorganic chemicals from the black liquor to be recycled in a pulping process. The third is to combust the organic constituents in the black liquor to produce valuable steam. For example, the efficient recovery of the inorganic pulping chemicals and the efficient generation of steam from the chemical recovery boiler are both essential elements in the economic and environmental aspects of the kraft pulping process.
Chemical recovery units, used in the pulp industry, utilize the liquor obtained from the digestion of wood or other cellulose material with certain chemicals. The liquor in such processes is sprayed into the furnace of the unit. The combustible portion of the liquor is burned and the chemicals in the liquor are smelted and drawn off of the lower end of the furnace. In typical chemical recovery units the heat that is evolved by burning the liquor is used to reduce the sulfur compounds and to generate steam by passing the combustion gas over suitable heat exchange surfaces.
This liquor often has a substantial moisture content. Most of the moisture is driven from the liquor spray upon the introduction of the liquor into the furnace because of the high temperature in the furnace. The hot gases originating from the bottom of the furnace will pass upwardly through the furnace. The solid particles fall onto the furnace hearth and form a pile. During the descent to the hearth some of the volatile substances are driven from these solid particles. The combustible material in the solids is burned in the pile that forms on the hearth. This combustion is supported by the introduction of preheated primary air which is directed generally over and upon this pile of material. The volatile matter and the combustibles contained in this solid material are burned. The remaining inorganic chemicals are melted and the sulfur compounds are reduced to sodium sulfide. Only the non-combustible material which includes the chemicals that are to be recovered along with the traces of various impurities are removed through a suitable spout.
In addition to primary air referred to above it is common practice to provide secondary air and tertiary air to control the combustion process. The present invention has particular application to controlling the flow of such secondary and tertiary air.
In the operation of a slagging furnace, such as a chemical recovery furnace, the amount of combustion air required is proportional to the quantity of fuel being burned. Conventional nozzles through which air is introduced have no moving parts. They are fixed in number and fixed in area. In such nozzles, air velocity through the nozzle will therefore be a function of the quantity of the fuel being burned. This means that the penetration of the air stream from the respective nozzles at low combustion rates will be greatly reduced. This may result in odorous emissions and other consequences of poor air distribution and insufficient furnace turbulence may result.
Variable area nozzles known in the industry operate by closing off the top of the fixed nozzle by lowering a damper or guillotine. Such apparatus has not been wholly satisfactory for the following reasons:
1. Conventional dampers tend to be difficult to move because of the sliding friction involved in moving the damper. PA1 2. Conventional variable area nozzles have moving parts that are close to the furnace opening and thus tend to overheat and become jammed as the result of thermal distortion. PA1 3. The moving parts of conventional variable area nozzles are close to the furnace opening and thus are continually exposed to the hot ash and molten smelt. Accordingly, slag runs down the wall and freezes on the moving parts when the molten smelt and hot ash are exposed to the lower temperatures of the combustion air passing through the nozzle. PA1 4. Closing off the top of the furnace opening in conventional variable area nozzles allows the molten smelt to run down into the inactive area of the nozzle and freeze. As a result, the damper is locked into the closed portion by the frozen smelt. The frozen smelt is inaccessible to poke rods because the damper obstructs access. PA1 5. Thermal expansion of the air duct connecting the windbox to the nozzle acts to distort the nozzle shape which then causes binding of the control damper.
It is an object of the invention to provide apparatus which will be substantially insensitive to the effect of thermal expansion and contraction of the component parts.
Still another object of the invention to provide apparatus will have relatively little friction.
Another object of the invention is to provide apparatus which has moving parts which are located further away from the hot ash and molten material that are present in the interior of the furnace.
Still another object of the invention is to provide apparatus which will minimize pulling hot furnace gases into the nozzle and thus minimizes the occurrence of overheating and plugging as a result of pulling such gases into the nozzle.